The present invention relates generally to electrically pulsed, droplet deposition apparatus and more particularly to such apparatus in the form of a high density multi-channel array. A common use to which apparatus of this kind is put is as a drop-on-demand ink jet printhead.
A high density array printhead should clearly have the property that each channel can be actuated separately and that the energy applied to one channel is only minimally coupled into neighboring channels. Energy coupling between channels is often referred to as "crosstalk." In copending U.S. Pat. applications Ser. Nos. 140,764 now U.S. Pat. No. 4,879,568 and 140,617, now U.S. Pat. No. 4,887,100 both filed Jan. 4, 1988, and both entitled "Droplet Deposition Apparatus," there are disclosed ink jet printheads having a plurality of parallel channels mutually spaced in an array direction normal to the length of the channels. The printheads employ shear mode actuators, which occupy side walls of the channels, for expelling droplets from nozzles respectively communicating with the channels Shear mode actuators avoid one type of crosstalk; namely, crosstalk arising from volume changes in the actuators caused by elastic interaction from stress waves travelling through the piezoelectric material of the printhead. That is, shear mode actuators when actuated do not experience a volume change, for example, a change in length or height.
Actuation of two groups respectively of odd and even numbered channels in an alternate manner is a further feature of shared, shear mode wall actuators as disclosed in the previously mentioned copending application Ser. No. 140,617. In this type of system, the creation of a pressure p in a selected channel induces a pressure -p/2 in the immediate neighboring channels which therefore cannot be actuated at the same time as the selected channel. Pressure crosstalk,:namely energy coupling into the neighboring channels of the same group, also occurs when compliant channel wall actuators of the selected channel are actuated. This can be avoided by the offset form of channel arrangement disclosed in application Ser. No. 140,617.
Although crosstalk reduction has been effected in the ways described for the forms of crosstalk referred to, a further source of crosstalk has been identified which is troublesome and requires a different approach to accomplish its reduction. The shear mode wall actuators of a printhead of the kind referred to, when actuated, are subject to respective fields normal to electrodes on opposite sides of the channel facing walls which comprise the actuators. These fields give rise to fringe fields which, in the vicinity of the roots of the wall actuators, have significant components parallel to the poling direction so that the piezoelectric material in these regions is volumetrically distorted rather than being deflected in shear. The overall effect of these fringe fields is to deflect the base material at the roots of the wall actuators to induce crosstalk into the neighboring channels and at the same time to reduce significantly the wall actuator deflection.